1. Field of the Invention
The present invention relates to a fuel cell stack comprising a stack of sub-stacks, with intermediate plates interposed therebetween, each comprising a stack of membrane electrode assemblies, with separators interposed therebetween, each having an anode, a cathode, and an electrolyte sandwiched between the anode and the cathode.
2. Description of the Related Art
Solid polymer electrolyte fuel cells, for example, employ an ion exchange membrane (electrolyte) comprising a polymer ion exchange membrane (proton ion exchange membrane). A membrane electrode assembly comprises an anode and a cathode, each made up of an electrode catalyst and a porous carbon panel, that are disposed on the opposite sides of the ion exchange membrane. The membrane electrode assembly is sandwiched between separators (bipolar plates), making up a unit cell for generating electricity. A predetermined number of such unit cells are stacked for use as a fuel cell stack.
When a fuel gas, e.g., a gas mainly containing hydrogen (hereinafter referred to as “hydrogen-containing gas”) is supplied to the anode, the hydrogen in the gas is ionized on the electrode catalyst and moves through the ion exchange membrane to the cathode. Electrons are supplied to an external circuit, which uses the electrons as an electric energy of a direct current. Since the cathode is supplied with a gas mainly containing oxygen (hereinafter referred to as “oxygen-containing gas”), for example, hydrogen ions, electrons, and oxygen react with each other on the cathode, producing water.
If the fuel cell stack is to be used on motor vehicles, then it is required to produce a relatively large output. To meet such a requirement, it is customary to use a stack of many unit cells. As the number of stacked unit cells increases, however, a temperature distribution tends to occur along the stack, and the ability of the fuel cell stack to discharge generated water produced by an electrochemical reaction in the fuel cells is lowered, failing to provide a desired electric energy generating capability.
One known solution to the above problem is an apparatus disclosed in U.S. Pat. No. RE36,148. In the disclosed apparatus, as shown in FIG. 12 of the accompanying drawings, a fuel cell block 1 is divided into a first cell group 2, a second cell group 3, and a third cell group 4, which are stacked in the direction in which a reactant gas, e.g., a fuel cell, is supplied, i.e., the direction indicated by the arrow a. The first through third cell groups 2, 3, 4 have respective numbers of unit cells 5a, 5b, 5c. 
The fuel cell block 1 is supplied with the reactant gas through a line 6. The reactant gas is first supplied concurrently to the unit cells 5a of the first cell group 2. After having been discharged from the first cell group 2, the reactant gas is supplied concurrently to the unit cells 5b of the second cell group 3. Thereafter, the reactant gas is discharged from the second cell group 3 and supplied concurrently to the unit cells 5c of the third cell group 4. With the disclosed arrangement, it is possible to effectively discharge produced water and an inactive gas from the fuel cell block 1 and to increase the electric energy generating capability of the fuel cell block 1.
In the fuel cell block 1, the reactant gas flows in alternately opposite directions in the first through third cell groups 2, 3, 4, and the first through third cell groups 2, 3, 4 need to have different separator structures. Therefore, the fuel cell block 1 requires an increased number of different types of separators, and is not economical due to the relatively high cost of manufacturing required separators.
When the ion exchange membranes of a fuel cell stack are dried, the fuel cell stack is unable to operate at a high output density. Therefore, it is necessary to humidify the ion exchange membranes while the fuel cell stack is in operation. There have been proposed various processes for humidifying the fuel cell stack. The proposed humidifying processes include an external humidifying process, an internal humidifying process, and a self-humidifying process. According to the external humidifying process, a humidifier such as a bubbler or the like is provided outside of the fuel cell stack, and a reactant gas is humidified by the humidifier to supply moisture to a membrane electrode assembly for thereby humidifying ion exchange membranes in the membrane electrode assembly. According to the internal humidifying process, each unit cell has a humidifier (humidifying structure) incorporated therein for humidifying ion exchange membranes in the membrane electrode assembly. According to the self-humidifying process, which is a type of the internal humidifying process, ion exchange membranes in the membrane electrode assembly are humidified by water generated as a result of an electrochemical reaction in the ion exchange membranes.
The external humidifying process makes the entire fuel cell assembly large in size and causes the fuel cell assembly to take up a large space because the humidifier is required as an additional device outside of the fuel cell stack. The external humidifying process is also disadvantageous in that the humidifier may suffer a follow-up capability problem when the load on the fuel cell stack is abruptly increased.
The internal humidifying process includes a humidifying process which employs water absorption fibers embedded in ion exchange membranes, a humidifying process which employs water permeable plates extending from anodes, and a humidifying process which employs water absorption fibers held in contact with anode sides of ion exchange membranes. These humidifying processes, however, are problematic in that the ion exchange membranes cannot easily be repaired in case they are not sufficiently humidified for some reasons.
The self-humidifying process includes a humidifying process which employs fine particles of platinum dispersed in ion exchange membranes for generating water due to a reaction between hydrogen and oxygen gases which flow in from anodes and cathodes, and a humidifying process which employs very thin ion exchange membranes for passing water produced in cathodes therethrough to anodes. These humidifying processes are highly costly to carry out because they need special ion exchange membranes, and are problematic in that it is difficult to produce ion exchange membranes having desired characteristics.
It has been proposed to use the technical concept of a solid polymer electrolyte fuel cell disclosed in Japanese laid-open patent publication No. 10-284095 in a structure for humidifying ion exchange membranes.
According to the proposed scheme, as shown in FIG. 13 of the accompanying drawings, a separator 7 of a solid polymer electrolyte fuel cell has a reactant gas inlet 8a and a reactant gas outlet 8b which are defined through the separator 7. The separator 7 also has a plurality of gas flow grooves 9 defined in a surface thereof in communication with the reactant gas inlet 8a and the reactant gas outlet 8b. 
The separator 7 further includes an auxiliary inlet 8c defined therethrough in communication with intermediate portions of the gas flow grooves 9. A reactant gas introduced from the reactant gas inlet 8a into the gas flow grooves 9 is consumed as it flows through the gas flow grooves 9, combined with a dry reactant gas supplied from the auxiliary inlet 8c, and finally discharged from the reactant gas outlet 8b. 
Since the reactant gas supplied from the auxiliary inlet 8c is dry, it lowers the partial pressure of water vapor contained in the reactant gas flowing through the gas flow grooves 9, thus preventing moisture from being condensed and hence preventing condensed water from being deposited and retained on wall surfaces of the gas flow grooves 9. The disclosed arrangement serves to provide a stable fuel cell capability.
The disclosed fuel cell structure may be used in a different condition as follows: The reactant gas inlet 8a is supplied with a reactant gas and moisture in an amount required to cause a reaction in an upstream region of the gas flow grooves 9, and the auxiliary inlet 8c is supplied with a low-humidify reactant gas in an amount required to cause a reaction in a downstream region of the gas flow grooves 9. Water generated when the reactant gas flows in the upstream region of the gas flow grooves 9 is used to humidify the low-humidify reactant gas supplied from the auxiliary inlet 8c, making it possible to supply the downstream region of the gas flow grooves 9 with a required amount of reactant gas and moisture. As a result, the amount of humidifying water may be reduced, and the entire fuel cell structure may be simplified and reduced in size.
When the separator 7 is in use, since the auxiliary inlet 8c supplies a dry reactant gas (low-humidify reactant gas) directly to the gas flow grooves 9, the humidified reactant gas flowing through the gas flow grooves 9 and the dry reactant gas may not be uniformly mixed with each other. Consequently, the humidify and concentration of the reactant gas supplied from the gas flow grooves 9 to electric energy generating regions tend to vary, resulting in a reduction in the electric energy generating capability.